1. Field of the Invention
The present invention relates to a stencil base sheet, a method of producing a printing master from the stencil base sheet, and a method of performing monochrome or multi-color stencil printing, using the above-mentioned stencil base sheet.
2. Discussion of Background
Conventionally, when a printing master is made from a thermosensitive stencil bass sheet, thermal energy is applied to the stencil base sheet through a thermal head. In this case, pulse signals with a predetermined electric power and voltage are applied to the thermal head to form perforations in the stencil base sheet. Therefore, images obtained on an image receiving sheet using the printing master are printed in dots of the same size.
When a photographic image with a halftone is printed using such a thermosensitive stencil base sheet, if the perforation is performed in the stencil base sheet with the application of thermal energy based on a binary system, using a predetermined threshold of the optical density for the image, an image with an extremely high contrast can be obtained, but without the halftone, so that when such a photographic image is printed, a technique of exhibiting the halftone with the density of dots, for example, an error diffusion process represented by a dither method, is conventionally used. Such technique can improve the gradation of the image much better than the above-mentioned method using the binary system can. However, the above-mentioned technique is not always satisfactory due to the formation of some graininess in highlight portions, and insufficient resolving power for thin lines.
In order to solve the above-mentioned problems, Japanese Laid-Open Patent Application 08-90748 proposes that the energy to be applied to the thermal head be controlled by a pulse width or an applied voltage which is adjusted so as to be proportional to the image density, thereby obtaining perforations of different sizes corresponding to Image densities. This method can improve the quality of halftone images much more than the above-mentioned method using the binary system, but is not yet satisfactory since small perforations cannot be accurately formed, and that some graininess is still observed, in particular, in highlight portions.
To be more specific, even when the stencil base sheet is prepared by gluing a thermoplastic resin film to a porous support, if the thermoplastic resin film cannot compensate for the roughness of the porous support, the roughness of the porous support evidently appears at the surface of the thermoplastic resin film, and the surface smoothness of the thermoplastic resin film is lowered. If this takes place, a heat-emitting resistor element such as a thermal head cannot sufficiently come into close contact with some portions of the stencil base sheet. In such portions, heat transfer from the heat-emitting resistor element to the stencil base paper is insufficient, so that perforations cannot be accurately formed in the portions. If the diameter of each perforation differs depending upon the state of the contact of the heat-emitting resistor element with the stencil base paper, even though the energy applied to the heat-emitting resistor element is accurately controlled so as to have a pulse width or a voltage in proportion to the image recording density, it is impossible to control the size of each perforation according to the applied energy. As a result, dots with a predetermined area cannot always be accurately formed. In particular, at highlight portions, it is required that small perforations be accurately formed with the application of a small amount of thermal energy thereto. Therefore, when the heat-emitting resistor element is not in close contact with the stencil base sheet, a number of imperfect or inappropriate perforations will be formed, so that there may occur a problem that no dots are formed in the portions whore there should be formed dots, that is, the so-called "dots omission" problem takes place. The result is that the graininess of the highlight portions is increased and the image quality is significantly lowered.
Japanese Laid-Open Patent Application 5-212983 discloses a stencil base paper comprising a thermoplastic resin film with a smoothness of 1000 seconds or more, which is prepared in order to improve image quality, perforation sensitivity and transportation performance of the stencil base paper.
Japanese Laid-Open Patent Application 8-67081 also discloses a stencil base sheet comprising a thermoplastic resin film which has a maximum surface roughness of 10 .mu.m or less and a surface smoothness of 1000 seconds or more in order to improve the image quality and perforation sensitivity.
These stencil base sheets, however, are not satisfactory, either for the formation of perforations by the image formation based on the above-mentioned binary system without pulse modulation due to the formation of the graininess in highlight portions, or for the formation of perforations by a multi-valued dot image formation with pulse modulation, using a thermal head.
Furthermore, when a uniform solid image is formed on an image receiving sheet using the stencil base sheet, there may occur a so-called "offset" that an unfixed toner which stays on a previously discharged image receiving sheet without being absorbed by the image receiving sheet comes in contact with the back side of the next discharged image receiving sheet and the back side of the image receiving sheet is stained with the unfixed ink. This problem is caused by the ink being transferred to the solid image portion of the image receiving sheet in an amount more than the amount that can be absorbed by the solid image portion.
In order to prevent the occurrence of the above-mentioned "offset", the following methods are disclosed:
In Japanese Laid-Open Patent Application 1-267094, there is disclosed a thermosensitive stencil base paper comprising a support which is composed of at least two paper layers with different densities made of tissue papers.
In Japanese Laid-open Patent Application 4-265783, there is proposed a method for producing a printing master from a stencil paper in such a manner that the peripheral portion of each perforation made in the printing master protrudes from the other portions of the printing master.
Furthermore, in Japanese Laid-Open Patent Application 8-197825, there is disclosed a stencil master comprising a porous sheet having such ink penetration routes that hinder direct flow of the ink to prevent the occurrence of the offset. However, the "offset" problem is not completely solved by the above-mentioned methods.
Japanese Laid-Open Patent Application 2-16053 discloses a stencil printing method in which an excessive ink is removed from printed materials by use of an excess-ink-transfer sheet.
Japanese Laid-Open Patent Application 8-239613 discloses an emulsion ink for stencil printing, which comprises a compound whose phase is changed to a liquid at a temperature in the range of 30 to 100.degree. C. in an oil phase of the emulsion ink.
By the above-mentioned methods and the ink, the "offset" problem can be solved to some extent, but cannot be completely solved. Furthermore, the above-methods have a shortcoming that special apparatus is required.